The present invention relates to an apparatus for amplifying a signal, preferably to obtain a sine wave which is suitable for a ringer. More particularly, the invention relates to a low cost, minimal power loss and self-adjustable power amplifier.
In many telephony applications, including fixed wireless access and pair gain systems, it is necessary to generate a ringer voltage with a sinusoidal wave form. Typically the sine wave has a voltage amplitude of between 40 volts RMS (Root Mean Square) and 75 volts RMS with a DC offset. The sine wave frequency is generally 20, 25 or 50 Hertz.
In designing a sine wave generator, key considerations include low cost with standard parts, minimal power loss, and ability of generator to meet specified requirements under varying load and input voltage conditions. Feedback is a preferred method of ensuring correct output in many applications including sine wave generation. Shimizu (U.S. Pat. No. 5,229,929) demonstrates the use of feedback for purposes other than sine wave output generation. Output peak current correction is performed using a feedback signal which indicates the portions of the output waveform that are outside a predetermined range of amplitude.
FIG. 1 shows one type of prior art apparatus 63 for generating a sine wave suitable for ringer. A linear amplifier 64 with output power stage of class B (according to a classification known to those skilled in the art) receives a reference signal 132 being a small accurate sine wave with a DC offset. The amplifier 64 compares the reference signal with a feedback signal 82 of an output 134. The gain of output 134, with respect to reference wave 132, is set by circuit elements 136 and 138. DC supply voltage outputs 140 and 142 are generated from the conversion of DC input 144 by a DC/DC converter 146. Chen (U.S. Pat. No. 5,600,713) describes an example of this type of generator. The apparatus 63 does contain a feedback, but it is power inefficient since 25% of its power is lost in the power stage due to the operation in the linear region that is typical for class B amplifiers.
FIG. 2 shows a second type of prior art apparatus for generating a sine wave suitable for a ringer, using a class D switching amplifier 100. An upper transistor 102 and a lower transistor 104 thereof switch on and off as in a conventional switching power supply, but at a fixed predetermined rate. Upper transistor 102 forms the positive part of the output sine wave 106, while the lower transistor forms its negative part. The output voltage 106 is filtered from high frequency components by a low pass L-C filter 90 containing inductance 92 and capacitance 94, to give a final output curve 108. The frequency and the form of the final output 108 (usually 20, 25 or 50 Hz) is determined by a predetermined timing table block 84. DC voltages 86 and 88, which are fed to a class D switching amplifier 100, determine the amplitude of final output 108. DC voltages 86 or 88 are generated from the conversion of DC input 96 by DC/DC converter 98. The apparatus 83 does not include a feedback loop to control the output signal, which is kept approximately sinusoidal by a predetermined timing table block 84.
Wendt (U.S. Pat. No. 5,307,407) illustrates a variation of the ringer of the second type which does include a feedback loop. The generation of output ringer signal is not, however, coupled to the generation of the power supply voltages, as the feedback loop only impacts the duty cycle of the switches of the ringer output and does not control the magnitude of the power supply voltages which are separately generated. The efficiency of each separately taken unit, namely the high voltage power supply and the ringer circuit, is high, but each unit inevitably contributes to reduce the overall efficiency. Additionally, there is an added expense and complexity in the combination of the two sets of switches.
FIG. 3 illustrates a third type of prior art apparatus 110 for generating a sine wave suitable for a ringer. A switch 114 is turned off and on by a predetermined timing table block 112, controlling the voltage applied to the transformer 116. The resulting power supply voltages 118 and 120 are in the form of full wave rectified sine waves generated by DC/rectified sine wave converter 130 from a DC input voltage 113. The form and frequency of supply voltages 118 and 120 are established by the predetermined timing table block 112, and the amplitude of these supply voltages is dependent on input voltage 113 and the output load current. Voltages 118 and 120 are unfolded into sine waves by means of an unfold circuit through output transistors 124 and 126 to give the output ringer signal 128. Apparatus 110 is more cost effective and efficient than apparatus 83 because it uses only one step of power conversion, however it does not include a feedback loop to control the output voltage 128, and the voltage is kept approximately sinusoidal by a predetermined timing table block 112.
There is thus a widely recognized need for, and it would be highly advantageous to have, a sine wave generator having minimal power loss, which integrates the generation and correction of the output signal and power supply voltages.